Myeloblasts

Myeloblasts are the earliest recognizable precursor in the granulocytic series, arising from the common myeloid progenitor in the bone marrow. Morphologically, they are large cells with fine, lacy chromatin, prominent nucleoli (typically 2-4), high nuclear-to-cytoplasmic ratio, and scant basophilic cytoplasm that may contain Auer rods—needle-shaped azurophilic granule aggregates composed of myeloperoxidase, which are pathognomonic for myeloid lineage (especially acute promyelocytic leukemia, AML M3). Myeloblasts differentiate sequentially into promyelocytes, myelocytes, metamyelocytes, band cells, and finally mature segmented neutrophils. Normally, myeloblasts constitute less than 5% of nucleated bone marrow cells and are essentially absent from peripheral blood. Clinically, an increase in myeloblasts is significant: 20% or more blasts in bone marrow or peripheral blood is the WHO diagnostic threshold for acute myeloid leukemia, reflecting a maturation arrest with uncontrolled proliferation of immature myeloid cells. This leads to bone marrow failure with resultant anemia, thrombocytopenia, and neutropenia (despite elevated total WBC count due to blast proliferation), causing fatigue, bleeding/bruising, and infections. Myeloblasts stain positive for myeloperoxidase (MPO) and Sudan black B, which helps distinguish AML from acute lymphoblastic leukemia (ALL), where lymphoblasts are MPO-negative and instead express TdT. Flow cytometry further characterizes myeloblasts using surface markers such as CD13, CD33, CD34, and CD117 (c-KIT). Chromosomal abnormalities such as t(15;17) (PML-RARA, APL), t(8;21), or inv(16) are associated with specific AML subtypes and have prognostic and therapeutic implications (e.g., all-trans retinoic acid for APL).